testsuite: Fix gdb.gdb/selftest.exp for C++-O2-g-built GDB

testsuite: Fix gdb.gdb/selftest.exp for C++-O2-g-built GDB

tested on Fedora 24 x86_64 after:
        ./configure; make
That is: CFLAGS='-g -O2' CXXFLAGS='-g -O2'

FAIL: gdb.gdb/selftest.exp: unknown source line
FAIL: gdb.gdb/selftest.exp: step into xmalloc call

gdb/testsuite/ChangeLog
2016-07-20  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.gdb/selftest.exp (do_steps_and_nexts): Add "next over TRY" and
	"step into captured_main (args)".